With the MOS transistors of CMOS integrated circuits being made smaller, it has been found necessary to make the source and drain junctions of such transistors very shallow. It has been found that standard methods of doping the source and drain regions, such as by diffusion or ion implantation, cannot produce junctions which are both sufficiently shallow and have the necessary concentration of conductivity modifiers. It has been shown that semiconductor junctions as shallow as 50 nanometers can be made using a doped refractory metal silicide as a diffusion source. However, the application of this technique to submicron channel length CMOS integrated circuits raises a number of problems.
One technique which has been used to selectively form a refractory metal silicide on a bare silicon surface but not on insulating material adjacent the bare silicon surface is to deposit the refractory metal over both the exposed silicon surface and the surface of the insulating material adjacent the exposed surface. The device is then heated to a temperature at which the metal on the exposed silicon surface reacts with the silicon to form the silicide. However, there is not enough silicon in the oxide layer to combine with the metal thereon to form a silicide. Thus, the silicide is selectively formed only on the bare silicon surface. The metal remaining on the oxide layer can then be removed with a selective etch. The silicide can then be doped so as to be a diffusion source for forming a junction in the silicon body. A problem with this technique is that when forming the silicide, silicon ions diffuse very rapidly into the metal so that the edge of the silicide is not well defined. If the space between the bare silicon areas, such as the channel region of a MOS transistor, is very small, silicide stringers can be formed during the silicidization step which could bridge the space between the adjacent silicide areas. Small silicide strips could be as large as 1 micrometer or larger. Thus, for submicron MOS transistors wherein the distance across the channel region is less than 1 micrometer, the process is not reliable.
Another technique which has been used to selectively form a refractory metal silicide on a bare silicon surface is to use a refractory metal which can be selectively deposited on bare silicon and will not deposit on an oxide layer. Tungsten, molybdenum and tantalum can be so selectively deposited using a low pressure chemical vapor deposition process. The refractory metal can thus be deposited only on the bare silicon surface where it can be silicided upon heating. A problem with this method is that the selective deposition process consumes silicon. For example, a 1 nanometer layer of tungsten deposited onto bare silicon will consume 2 nanometers of silicon. As a result the surface of the silicon is lowered by the thickness of the tungsten deposited. At the edge of the bare silicon surface, the tungsten actually tunnels under the adjacent oxide layer. After silicidization, implantation and diffusion for shallow junctions, the junction depth at the edge regions is extremely shallow. The radius of curvature at the edge of the junction is very small. In addition, there is stress at the silicide/silicon interface. Also, since the breakdown voltage of this shallow junction is very small, the drain breakdown voltage for junctions made with this method will be small.
One attempt to overcome the above problems was to selectively deposit a thin layer of single crystalline silicon onto the source and drain regions prior to depositing the refractory metal and siliciding it. This eliminates the shallow junction formation and breakdown problems. However, the control of very shallow effective drain junctions is not easy, and, if a drain extension is desire, the process becomes even more difficult to control.